The promised performance of wide band gap electronic devices (e.g. GaN, SiC, and GaAs based devices) will result in much high power dissipation and localized heat generation at contacts and in channel regions than can be accommodated by current state-of-the-art thermal management configurations. As a consequence, use of conventional cooling techniques imposes a ceiling on wide band gap device performance and reliability. Overcoming such barriers requires thermal engineering at the macro, micro, and nano-scale, which can provide significant reductions in the near-junction temperature rise and component thermal resistance.
Specific challenges relate to heat spreading in certain types of radio frequency (rf) power devices. In such devices the local power densities can exceed, for example, 1 MW/cm2. Spreading this heat and lowering the junction temperature enables increased reliability and also continuous wave performance. In addition to electronic device applications, there is also a need to improve upon current state-of-the-art thermal management configurations in certain extreme optical applications.
Synthetic diamond materials have been proposed as an ideal solution in extreme thermal management applications due to the high in-plane thermal conductivity of such materials. For example, various grades of synthetic diamond material grown by chemical vapour deposition (CVD) are already commercially available for thermal heat spreading applications including both polycrystalline and single crystal synthetic diamond materials.
One problem with using diamond materials in such application is that diamond materials can be difficult to bond to other components and this is a particular issue when used in high power density applications where any bonding will be subject to large changes in temperature and where the thermal barrier resistance becomes critical to device performance. Metallization of diamond heat-spreaders is required to provide a wettable surface for die-attachment. While diamond is known to be an excellent room temperature heat spreader, its usefulness in reduction of device junction temperatures is reduced by the TBR (thermal barrier resistance) associated with its method of attachment to electrical devices.
Typically, for reasons of adhesion and mechanical robustness, three-layer metallization schemes are used for bonding of diamond components. An example of such a three-layer metallization scheme comprises: (i) a carbide forming metal layer which forms a carbide bonding to the diamond component; (ii) a diffusion barrier metal layer disposed over the carbide forming metal layer; and (iii) a surface metal bonding layer disposed over the diffusion barrier metal which provides both a protective layer and a solderable/wettable surface layer onto which a metal solder or metal braze can be applied to bond the diamond component to another device component. A particular example of such a three-layer metallization scheme is Ti/Pt/Au and typically diamond heat spreading components are sold in metallized form coated with such a three-layer metallization structure such that other components can ready be mounted and bonded to the diamond component using a solder or braze.
In general, as well as mechanical attachment, the layered metallization coating should be consistent with low thermal barrier resistance, such as to maximize the effectiveness of a diamond heat spreader bonded to a high power density semiconductor component.
Across a diamond-metal-semiconductor interface in such an application, heat transport is a non-trivial physics process, alternating between phonon and electron transport processes with different scattering mechanisms. Despite the actual thickness of some of the formed interfaces being very low, for example the titanium layer forming a titanium carbide bond with the diamond surface can be very thin (only tens of nanometers), these interfaces can add considerably to the thermal resistance and hence effectiveness of the thermal solution and this is particularly problematic in high power density applications.
Following on from the above, the present inventors have revisited the problem of mounting and bonding diamond components with the aim of providing a bonding methodology which has improved functional performance in terms of lower thermal barrier resistance when used in high power density applications.